1. Technical Field
A fluorescent image, produced by ultraviolet or soft x-ray irradiation of a fluorescent crystalline material is focused by use of conventional glass optics. Choice of material emitting in the visible spectrum permits viewing by the unaided eye. Highest resolution results from electronic processing of fluorescent images at ultraviolet wavelengths.
2. Description of the Prior Art
Optical microscopy is capable of resolution smaller than the wavelength of imaging light. The ultimate resolution of a microscope is defined by the `incoherent cutoff spatial frequency`. It corresponds to a grating with lines and spaces of dimension: EQU Resolution=(K.sub.1 .multidot..pi./NA Eq. 1
where NA is the numerical aperture of the microscope objective and K.sub.1 is a dimensionless constant. At the cutoff frequency, K.sub.1 =0.25.
The "oil immersion" microscope avoids one obstacle to high resolution. It avoids reflections by use of a refractive index matching fluid between the object being studied and the objective lens. Its use permits resolution associated with attainment of NA values of 1.5 and higher. As an example, viewed radiation of wavelength .lambda.=0.4 .mu.m, for NA=1.6, permitting ultimate resolution of 0.0625 .mu.m, is attainable. This ultimate resolution corresponds to zero contrast. Useful resolution, for a still-low contrast image is 1.2.times.0.0625 .mu.m=0.075 .mu.m. A resolution value of about 1.5.times.0.0625 .mu.m or 0.1 .mu.m assures sufficient contrast for most purposes.
This known capability has provoked use of conventional glass optics for resolving features ordinarily considered the province of UV or x-ray. One effort uses image conversion from short wavelength to the visible or near-visible. The long wavelength image is produced by illumination of a fluorescent material. The approach has not gone into general use. The fluorescent material has been the problem. The obvious choice--the familiar cathode ray tube phosphors have proven unsatisfactory. Granularity of the powder films limits resolution. Thin fluorescent polymer films, in which fluorescence is produced by dissolved fluorescent species, does avoid granularity. Such polymer films, while producing high resolution images, are limited by brightness (and, therefore, by contrast). Increasing solute concentration increases brightness in initial use, but the films become unstable (and grainy) due to precipitation.